ÚJV Řež, a. s.

The need for strengthening of

international cooperation in the area

of analysis of radiological

consequences

Jozef Misak

IAEA Technical Meeting on Source Term Evaluation of Severe Accidents

21 – 23 October 2013, Vienna

1

Content

• The presentation summarizes the reasons for harmonization

of acceptance criteria and methodology for assessment of

radiological consequences of reactor accidents for various

applications and provides relevant recommendations for the

IAEA actions

2

Reference documents

• Safety Assessment for Facilities and Activities, GSR Part 4, IAEA (2009)

• Safety of NPPs: Design, SSR-2/1, IAEA (2012

• Deterministic Safety Analysis for NPPs, SSG-2, IAEA (2009)

• Safety Assessment and Verification for NPPs, NS-G-1.2, IAEA (2001)

• Format and Content of SAR, GS-G-4.1, IAEA (2004)

• Reactor Harmonization Group, WENRA Reactor Safety Reference Levels, January 2008

• WENRA, Reactor Harmonization Group, Reactor Safety Reference Levels, January 2008

• European Utility Requirements for LWR NPPs. Rev. C, April 2001

3

Applicability of radiological analysis

• Radiological analysis provides inputs for

various documents developed and submitted

for regulatory review in different stages of

NPP life time, including

o Different stages of Safety Analysis Reports

o Environmental Impact Assessment

o Emergency Preparedness and Response Programme

o Environmental Monitoring Programme

4

Importance of harmonization

Radiological consequences

• Represent the direct measure of the level of safety

• Are publicly sensitive issues and therefore influencing public trust

• Have trans-boundary effects and implications

• Are cross-cutting elements contained in several documents of the safety case

• Are important for international comparison of different reactor designs

⇒ International harmonization of approaches to

determination of radiological consequences is

needed

5

Indication of areas where

harmonization would be appropriate

• Differences in radiological acceptance criteria for design basis accidents

• Absence of radiological acceptance criteria for severe accidents

• Differences in methodology for demonstration of compliance with the criteria

• Internal inconsistencies in IAEA Safety Standards

• Differences between methodologies in IAEA Safety Standards and other documents (such as WENRA Reference Levels or European Utility Requirements)

• Differences in methodologies used in various licensing documents (EIA, SAR)

6

Large differences in radiological

acceptance criteria for design

basis accidents

IAEA SSR-2/1 on high level criteria

Requirement 5: Radiation protection: The design of a nuclear power plant shall be such as to ensure that radiation doses to workers at the plant and to members of the public do not exceed the dose limits, that they are kept as low as reasonably achievable in operational states for the entire lifetime of the plant, and that they remain below acceptable limits and as low as

reasonably achievable in, and following, accident

conditions.

5.25. The design shall be such that for design basis accident conditions, key plant parameters do not exceed the specified design limits. A primary objective shall be to manage all design basis accidents so that they have no or only minor radiological impacts, on or off the site, and do not necessitate any off-site intervention measures.

Examples of radiological acceptance criteria for

DBAs – USA, Spain, Sweden, Korea, Japan, ...

AccidentEffective dose limit (at exclusion area

boundary)

LOCA 250 mSv

SGTR 25-250 mSv depending on additional conditions

Main steam line break25-250 mSv depending on additional conditions

Locked rotor accident 25 mSv

Rod ejection accident 63 mSv

Fuel handling accident 63 mSv

Small LOCA25-250 mSv depending on additional conditions

Gas waste system failure 1 mSv

DEC or severe accidents No limit established

Germany. Slovakia 50 mSv effective dose for all DBA

UK, Switzerland, Netherlands (in some cases depending on frequency):

100 mSv for frequency less than 1E-4/r.y

In addition to different numbers attention should be paid to the fact that limits are prescribed for:

Different duration of exposureDifferent pathways of exposureDifferent levels of conservatism in dose estimate

=> In many cases the criteria are too different and too large (not in compliance with IAEA Safety Standards), the first intervention level (sheltering) being ~ 10 mSv in 2-7 days

Examples of radiological acceptance criteria for

DBAs – Germany, Slovakia, UK, Switzerland,

Netherlands, ...

Acceptance criteria for radioactive releases / max doses to general public (STUK, Finland)

DBC 1, Normal operationradiation dose limit 0,1 mSv / year for the entire site

DBC 2, Anticipated events (f>1.E-2)radiation dose limit 0,1 mSv

DBC 3, Class 1 postulated accidents (1E-3 < f < 1E-2)radiation dose limit 1 mSv

DBC 4, Class 2 postulated accidents (f<1E-3)radiation dose limit 5 mSv

DEC, Design extension conditions, without core meltradiation dose limit 20 mSv

11

Absence of radiological

acceptance criteria for severe

accidents

IAEA SSR-2/1 on high level criteria

Requirement 5: Radiation protection: The design of a nuclear power plant shall be such as to ensure that radiation doses to workers at the plant and to members of the public do not exceed the dose limits, that they are kept as low as reasonably achievable in operational states for the entire lifetime of the plant, and that they remain below acceptable limits and as low as reasonably achievable in, and following, accident conditions.

5.31. The design shall be such that design extension conditions that could lead to significant radioactive releases are practically eliminated (see footnote 1); if not, for design extension conditions that cannot be practically eliminated, only protective measures that are of limited scope in terms of area and time shall be necessary for the protection of the public, and sufficient time shall be available to implement these measures.

13

Examples of acceptance criteria for

severe accidents

• No quantitative radiological acceptance criteria established in majority of countries (Czech Republic, Slovakia, France, Germany, USA, Russia, etc)

• Requirements considered fulfilled if release is not more than 0,1 %

of the core inventory of the caesium isotopes 134 and

137, contained in a reactor core of 1800 MWth (Sweden)

• Maximum release of Cs 137 100 TBq (Finland)

• Atmospheric release of caesium-137 below 30 TBq and the combined fall-out of nuclides other than caesium-isotopes shall not cause, in the long term, starting three months from the accident, a hazard greater than would arise from a caesium release corresponding to the above-mentioned limit (EUR, Bulgaria)

• EUR targets for short term and long term actions and for

limited economic impact

EUR Targets for short term protective

actions

Duration Distance Target Objective and observation

Ta

rget

fo

r

emer

gen

cy a

ctio

ns

7 days from the

release

initiation from

the plant

Any

distances

Effective

dose

committed

50 mSv

It’s intended to assure that at any distances

from the plant, emergency evacuation of the

public is not required. Cumulated releases

during first 24 hours of accident are

considered. Exposure by irradiation from the

plume, from deposits and from inhalation

should be considered (not from ingestion).

Ta

rget

fo

r

del

ay

ed a

ctio

ns

First 30

consecutive

days after the

release

termination

Beyond 3

km

Effective

dose

committed

30 mSv

It is intended to assure that beyond 3 km from

reactor public evacuation within 30 days after

termination of release is not required.

Cumulated releases during first 4 days of

accident are considered. Exposure by

irradiation from the deposits and from

inhalation due to resuspension of deposits

should be considered (not from ingestion).

Targets for protective actions are individual dose limits

EUR Target for long term protective

actions

Duration Distance Target Objective and observation

Ta

rget

fo

r lo

ng

-ter

m

act

ion

s Up to 50 years

from the

termination of

all releases

Any

distances

Effective

dose

committed

100 mSv

It is intended to assure that at any

distances from the plant public

relocation after the release

termination is “never” required.

Exposure by irradiation from the

deposits and from inhalation due to

resuspension of deposits should be

considered (not from ingestion).

Targets for protective actions are individual dose limits

EUR Targets for economic impact

Duration Distance Target Objective and observation

1st

Ta

rget

fo

r

Eco

no

mic

Im

pa

ct

After 1 year

from the end of

the accident

Beyond 10 km 1250 Bq/kg for

Cs137

2000 Bq/kg for

I131

750 Bq/kg for

Sr90

This land contamination

limit would allow free

trading of crops cultivated

beyond the said distance

from the reactor according

to existing EC regulations.

The limit is based on a

dose of 5 mSv to

individuals eating

contaminated food for 1

year.

2n

dT

arg

et f

or

Eco

no

mic

Im

pa

ct

After 1 month

from the end of

the accident

Beyond 100 km

Targets for economic impacts are land contamination limits

17

Differences in methodology for

demonstration of compliance

with criteria among the countries

18

Differences in methodology of analysis of

consequences among the countries

• Large difference in determination of core inventory fractions released under DBA conditions between the US (and Japan, Korea, or Spain) and majority of European countries

• USA (RG 1.183): the release of iodine and noble gases starts with gap inventory, continuing with releases from the fuel matrix, assuming that the core will melt and releases from the molten

corium will take place even in the case of DBA

• In many European countries it is assumed that only a fraction of fuel will fail releasing gap inventory to the RCS

• Conservative approach to prediction of the limited number of the fuel elements failed is used in accordance with the regulatory guidance documents in Finland, UK, France, Russia, Slovakia, etc.

19

Differences in methodology of analysis of

consequences among the countries

• EPR methodology: The assessment of released activity is based on conservative methods and assumptions (initial primary activity, rate of cladding failures, etc). The assumptions for calculating the radiological consequences (evaluation of doses) are set realistically in order to arrive at a reasonably conservative assessment of the radiological consequences

• Other methodological assumptions: the level of reactor coolant activity and the treatment of iodine spiking in DBAs, the inventory of fission products in the gap, forms of iodine and others

20

Inconsistencies in IAEA Safety

Standards

21

Safety Requirements and Safety Guides for design,

for safety assessment, for content of SAR

• GSR Part 4, art. 4.54.The aim of the deterministic approach (in safety analysis) is to specify and apply a set of conservative deterministic

rules and requirements…This conservative approach provides a way of compensating for uncertainties …

• GSR Part 4, Requirement 17: Uncertainty and sensitivity analysis shall be performed and taken into account in the results of the safety analysis and the conclusions drawn from it.

• SSR-2/1, art. 5.26. The design basis accidents shall be analysed in a conservative manner.

• SSR-2/1, art. 5.27: The effectiveness of provisions to ensure the functionality of the containment (in case of design extension

conditions) could be analysed on the basis of the best estimate

approach

• NS-G-1.2, art. 4.19: In general, the deterministic analysis for design

purposes should be conservative. The analysis of beyond design basis accidents is generally less conservative than that of design basis accidents.

22

• GS-G-4.1, art. 3.128. Deterministic analysis…It is acceptable that best estimate codes are used for deterministic analyses provided that they are either combined with a reasonably conservative selection of input data or associated with the evaluation of the uncertainties of the results.

• GS-G-4.1, art. 3.140. The analyses (of beyond design basis accidents) should use best estimate models and assumptions

and may take credit for realistic system action and performance, …

• SSG-2, art. 3.8. Although conservative assumptions and bounding analyses should be used for design purposes, more realistic analyses should be

used to evaluate the evolution and consequences of

accidents…

• SSG-2, chapter 9 specifically dealing with source term evaluation does not provide any guidance on the use of conservative vs realistic

analysis

• => Very limited and not always clear guidance on radiological analysis is provided in the IAEA Safety Standards

Safety Requirements and Safety Guides for design,

for safety assessment, for content of SAR

23

Differences between

methodologies in IAEA Safety

Standards and other documents

(such as WENRA Reference

Levels or European Utility

Requirements)

24

Conservative or best estimate analysis in

various documents

• GSR Part 4, art. 4.54.The aim of the deterministic approach

(in safety analysis) is to specify and apply a set of conservative deterministic rules and requirements…

• SSR-2/1, art. 5.26. The design basis accidents shall be analysed in a conservative manner.…

• WENRA Reference Levels, E 8.1: The initial and boundary conditions (in safety demonstration for design basis accidents) shall be specified with conservatism.

• WENRA Reference Levels, F 2.2: Realistic assumptions and modified acceptance criteria may be used for the analysis of the beyond design basis events.

25

Conservative or best estimate analysis in

various documents

• EUR, section 2.1.3.3 Deterministic analysis, item 14) ... For calculation of releases, physically-based assumptions and

best estimate evaluations, with suitable margins to

take into account uncertainties, are preferred.

• => there is not full consistency among

various documents

26

Differences in methodologies used

in various licensing documents

(EIA, SAR)

27

Inconsistency between radiological

analysis in different licensing documents

• In several countries, different licensing documents

reviewed by the same regulatory body are developed using different approaches to radiological analysis

• This is in particular true for Safety Analysis Reports and Radiological Environmental Impact Reports

• Even the analysis of the same accidents (DBAs, severe accidents) uses very different assumptions (e.g. scope of the core damage, weather conditions)

• Subsequently, the results of analysis in terms of doses

may differ by one or two orders

• Since both documents are publicly available, such different publicly sensitive information may be confusing for

the public

28

Conclusions

• Radiological assessment is a key component for overall assessment of safety of NPPs

• There are significant differences among the countries in acceptance criteria and methodology for assessment of radiological consequences, in particular in reactor accidents

• Acceptance criteria for DBAs are often not in compliance with

IAEA standards, criteria for severe accidents rarely defined

• The issue of radiological consequences is not sufficiently covered

in existing IAEA Safety Standards and other guidance documents

• Harmonization of criteria and methodology for radiological assessment can contribute to consistency of information about safety of NPPs

• It is understood that harmonization may be a difficult process due to the close interrelation of the issue with national legislation, but appropriate IAEA Safety Standards can become a driving force towards better harmonization

29

Recommendations for the IAEA

• More attention should be devoted by the IAEA to the issue of assessment of radiological consequences of reactor accidents

• In ongoing revisions of the IAEA Safety Standards attention should be paid to ensuring consistency and comprehensiveness in addressing radiological consequences in various standards

• Publication of relevant IAEA lower level technical documents should be accelerated

• Development of a specific Safety Guide (or updating of existing ones) on assessment of radiological consequences should be considered